There are numerous devices for delivering medicament on the market and also patented where the medicament is arranged in a container, such as a syringe, cartridge and the like, and wherein the medicament is exposed to pressure when it is to be delivered. A very common design is a generally tubular compartment having a stopper in one end of the compartment and a delivery member attached to the opposite end of the compartment, such as e.g. a needle, a nozzle or the like member capable of delivering medicament to a patient.
In order to deliver a quantity of medicament, the stopper is exposed to pressure, i.e. pushed into the compartment by a plunger rod, which could be done manually by a finger, which is the case for simple handheld syringes, or by pressure means such as springs, which is common in automatic or semi-automatic injectors.
In many instances it is desirable to be able to deliver a certain specified quantity of the medicament. This is for example the case with a multi-dose injection device, which is capable of delivering a number of specified, set, doses until the compartment is empty. One example is disclosed in the European patent application No. 05/104,734.8 where specific doses can be set before injection. The injection device disclosed is arranged with spring means for exerting a pressure on the medicament for delivering a specific dose, i.e. pushing the plunger rod and thus the stopper into the container.
The delivery of a dose requires a certain force from the spring means in order to overcome the friction between the somewhat resilient stopper and the inner surface of the cartridge and also to be able to press the medicament in liquid form through a rather small passage in the delivery member, possibly within a predetermined time.
Due to the elasticity of the components under pressure such as the stopper and also the medicament if non-newtonian, there is a prevailing pressure even when the stopper has been moved a predetermined distance and the dose has been delivered. This is in particular pronounced when handling medicament with rather high viscosity, medicament having resilient properties.
With this type of substance with high viscosity, and because very small passages of the delivery member often are used, a rather large force is required and because of the elasticity of the components, often a certain small quantity of the substance comes out of the delivery member even after performed delivery when the pressure is relieved, i.e. there is some dripping from the delivery member, which is unwanted, in particular when treating a patient and the substance may be dripping on the patient's skin, possibly causing irritation or inconvenient, undesirable effects.
One solution is disclosed in WO 2008/020,023 A1. The device of WO 2008/020,023 A1 comprises a mechanism which causes the plunger rod to move in the distal direction in order to release any remaining pressure after a dose delivery. The mechanism could comprise ratchets cooperating with each other or a “wave-shaped” surface on which rollers move, all in order to create a movement in the distal direction of the plunger rod.
The drawback with the solution according to WO 2008/020,023 A1 is that there are mechanical components that are to interact with each other, which on the one hand will cause noise and vibrations and on the other hand can lead to increased friction that has to be handled with larger and stronger springs.
The plunger rods of many medicament delivery devices are threaded and cooperate with threaded nuts whereby either the plunger rod is rotated or the nut is rotated when advancing the plunger rod. For disposable medicament delivery devices this solution works very well because when the plunger rod has moved to its most forward position, the medicament container is empty and the medicament delivery device can be discarded. However, for reusable medicament delivery devices which use an auto-mechanism for delivering a dose of medicament, there is a problem when using threaded plunger rods because they have to be threaded back to their original position. One example is disclosed in EP 0937471 A1 showing a pen injector where a threaded plunger rod is threaded back to its original position.
This operation is not appreciated by most users, and may also lead to wrong handling of the device in that there could be an uncertainty as to how far the plunger rod should be threaded back. Further, there are a number of medicament delivery devices where the delivery mechanisms, and mechanisms associated with the delivery, do not permit a return of the plunger rod.
There is thus room for developments regarding handling of pressure release situations with medicament delivery devices in order to avoid drooling.